Generalised Droop Control for Power Management in a Multi-Terminal HVDC SystemKamila Nieradzinska, Grain Adam,

W. Leithead and Olimpo Anaya-Lara

University of Strathclyde

• North Sea Connection• VSC-HVDC• Control strategy• DC-voltage droop control• Test system configuration • Results• Conclusions

Outline of Presentation

North Sea Connections

What is VSC

• VSC = Voltage Source(d) Converter

• Capacitor is normally used as energy storage

• VSC uses a self-commutated device such as GTO (Gate Turn Off Thyristor) or IGBT (Insulated Gate Bipolar Transistor)

• Power transfer over long distances

• Lower power losses compared to AC transmission

• Independent control over active and reactive power

• Voltage support

• Wind farm is decoupled from the onshore grid,

• Connected to the weak network

• Black start capability

Why VSC-HVDC…

Point-to-point Connection

G

Wind FarmVSC1 VSC2

T2T1C1

C2

C3

C4

HVDCGrid

Different control strategies employed for offshore wind farm and onshore grid.

Vector Control

• Three-phase rotating voltage and current are transformed to the dq reference frame

• Comparative loops and PI controllers are used to generate the desired values of M and and fed their values to the VSC

• Phase-locked-loop (PLL) is used to synchronize the modulation index.

+-

+-

PI

PI

+-

+-

PI

PI abc

dq

PLL

abc

dq

VSC converter

*A

*B

*di

*qi

dM

qM

di

qi

av

bv

cv

dqv

dqi

Outer controllers

inner controllers

The controlled parameters

from the system (P, Q, Vdc, Vac)

abci

v abc

Inner ControllerResponsible for controlling the current in order to protect the converter from overloading during system disturbances

Control Strategies – Inner Controller

qcd d sdv u Li v

cq q sqdv u Li v

PI+

-*sdi d

u

ωLsdi

PI+-

*sqi

qu

ωLsqi

cdv

cqv

sqv

sdv-

-

+

-

+

+

* *( ) ( )pi iid d d d du k i i k i i dt

* *( ) ( )q pi q q ii q qu k i i k i i dt

Outer controllerResponsible for providing the inner controller with the reference values, where different controllers can be employed, such as:

DC and AC voltage controllers

The Active and reactive power controllers

The frequency controller

Control Strategies – Outer Controller

**sd

sd

Pi

v

**sq

sd

Qi

v

* * *( ) ( )sd pdc dc dc idc dc dci k V V k V V dt

* * *( ) ( )sq pac ac ac iac ac aci k V V k V V dt

PI+-

*

dcV

dcV*

sdi

PI+-

*

acV

acV*

sqi

(a) (b)

x*P

sdv

*

sdi x

*Q

sdv

*

sqi

(a) (b)

Controllers Schematics

dq

abc

PWM

PI-ud

*

ωL

ωL

-+

isd

Md

vsd

isd

PI-uq

*-

+

vsd

isq

+

+

isq

+

-

PI-

+Mq

Inner

controllers

-+

*÷

PI

vsd

P*

ƒ

*ƒ

Outer

controllersV

V*

+

+

dq

abc

PWM

PIud

*

ωL

ωL

-+

isd

Md

vsd

isd

PIuq

*-

+

vsd

isq

+

+

isq

+

-

PI-

+Mq

Inner

controllers

-+ PI

*

Outer

controllersV

V*

Vdc

Vdc

+

+

Wind farm side VSC

Onshore grid side VSC

Active power and AC voltages control

DC and AC voltages control

DC Voltage Droop Control

The proposed droop control provides a reference voltage to the DC voltage controller ‘i’ taking into account the voltage at the support node ‘j’ as shown in equation:

Test System Configuration

G

G

G

B1

B2

B3

B4

B5

B6

B7

PCC1

PCC2

PCC3

Vdc+droop control

Vdc+droop control

Vdc(master)WF1

WF2

400MVA

400MVA

400MVA

400MVA

400MVA

400MVA

400MVA

VSC1

VSC2

VSC3

VSC4

VSC5

33kV/132kV500MVA

33kV/132kV500MVA

132kV/400kV500MVA

132kV/400kV500MVA

132kV/400kV500MVA

Power Balance – Droop Control ON

Time 0 - 1.5 1.5 - 3 3 – 4.5 4.5 - 6 6 – 7.5 7.5 - 9 VSC3 255 450 350 255 160 65 VSC4 255 175 220 255 295 330 VSC5 255 140 195 255 310 370

DC Voltage – Droop Control ON

Test System Configuration with Loss of VSC4

G

G

G

B1

B2

B3

B4

B5

B6

B7

PCC1

PCC2

PCC3

Vdc+droop control

Vdc+droop control

Vdc(master)WF1

WF2

400MVA

400MVA

400MVA

400MVA

400MVA

400MVA

400MVA

VSC1

VSC2

VSC3

VSC4

VSC5

33kV/132kV500MVA

33kV/132kV500MVA

132kV/400kV500MVA

132kV/400kV500MVA

132kV/400kV500MVA

Power Balance – Droop Control ON (No VSC4)

Time 0 - 1.5 1.5 - 3 3 – 4.5 4.5 - 6 6 – 7.5 VSC3 33.3 % 50 % 62.5 % 50 % 75 % VSC4 33.3 % 25 % 0 % 0 % 0 % VSC5 33.3 % 25 % 37.5 % 50 % 25 %

Conclusions

• The controller can respond to any power demand

• There are significant advantages in terms of power flow controllability

• This can prove to be very advantageous for connection of variable wind generation and assist in the power balancing of interconnected networks.